1. Field of the Invention
This invention relates to drug delivery devices and more specifically to implantable devices for delivering drugs.
2. Description of Related Art
Localized drug delivery has become increasingly important in applications such as tissue engineering, growth regulation, pain control, and therapeutic approaches to localized disease conditions including tumors, local nervous system conditions, and local vascular system conditions. The action of many drugs is greatly enhanced by, or in some cases requires, long-term local delivery of those drugs into the patient's body.
One method of localized drug delivery is injection of the drug at a local site at which the drug acts. Localized drug delivery may be combined with a slow release drug formulation. Disadvantages of localized drug injection include the difficulty of stabilizing the drug over the total release period and adequately controlling the concentration profile of the drug over time at the site of action during delivery.
Another method for localized drug delivery includes inserting a catheter to direct the drug to the desired body site, and using a pump to impel a drug through the catheter. An externally worn pump is conventionally used with an internally implanted catheter. However, the site of entry into the patient's body is prone to infection. In addition, the externally worn pump is bulky and inconvenient.
Implantable drug delivery devices, implantable pumps, for example, have been developed to address the disadvantages of techniques that use external pump and catheter systems. Implantable drug delivery pumps often include a reservoir for storing the drug, an injection port to enable injection of fresh drug preparations at regular intervals into the reservoir, and optionally a catheter for delivering the drug to the desired site. Application of fresh drug preparations avoids a problem of long term stabilization. One disadvantage of implantable drug delivery devices is an inherent difficulty in controlling the dosage and delivery rates from the reservoir through the catheter. A further problem is that many of the applications for implantable drug delivery utilize drugs that are not stable at body temperature for extended periods.
An implantable device known as the Duros implant, which is manufactured and sold by Alza Corporation, Mountain View, Calif., provides drug treatment by osmotically delivering the drug preparation into the patient. Osmotic delivery uses neither mechanical or electronic assistance so that the Duros implant device can be made very small, about the size of a few matchsticks. However, the Duros implant does not permit an easily adjustable delivery rate, cannot be refilled, and requires surgical implantation. In addition, the Duros implant has only a very small reservoir for storing the drug preparation.
The limited capacity reservoir may not be a significant problem for short-term delivery of drugs, but many patients are unwilling to undergo surgical implantation for only a short-term benefit. To use the limited capacity reservoir for long-term drug delivery, a concentrated, potent drug preparation must be delivered in small dosages over an extended delivery period, resulting in problems of long-term drug instability and low precision in dosage control.
Another type of implantable drug delivery device is an infusion pump, such as a SynchroMed Infusion System which is manufactured and sold by Medtronic, Inc., Minneapolis. The device includes a catheter and a pump section. The pump section further includes a collapsible reservoir and a fill port for refilling the reservoir with fresh drug preparation. SynchroMed automatically delivers a controlled amount of medication through the catheter using an electronically driven pump. The dosage, rate, and timing are programmed into electronics contained within the pump section from an external programming device. The external programmer transmits programming information using radio waves.
SynchroMed solves some long-term delivery and dosage accuracy problems of other devices, but the pump section is large and bulky, measuring approximately 8.5 cm in diameter and 2.5 cm in height, and weighing about 205 g. The size of the SynchroMed limits the flexibility of doctors in implanting the device. Because of the large size and awkward hockey-puck shape, the SynchroMed device must typically be implanted in the abdominal cavity of a patient, and an extended catheter is passed through the patient's body to deliver the drug to the desired site of administration. In addition to problems with size and placement, the SynchroMed device is burdened by complex electronics for both programming and pumping functionality.